The output stages of amplifiers and power inverters typically include a high-side transistor and a low-side transistor connected in a series path between a positive and negative voltage supplies or rails, "positive" and "negative" being used only in a relative sense. A common node between the high-side and low-side transistors is connected through a load resistor to a reference voltage (e.g., ground). As the high-side and low-side transistors are alternately switched on and off, the node fluctuates in the range between the positive and negative voltage rails and the current through the load resistor varies in magnitude and direction. The output of the amplifier or inverter is the voltage that appears across the load resistance.
A circuit diagram of a conventional audio amplifier 10 is shown in FIG. 1A. A complementary pair of transistors, including a high-side N-channel transistor Q.sub.A and a low-side P-channel transistor Q.sub.B, are connected in a series path between the positive voltage rail V+ and the negative voltage rail V-. The common node N between transistors Q.sub.A and Q.sub.B is connected through a load resistor R.sub.L to ground. The output voltage V.sub.OUT is taken across load resistor R.sub.L.
The drive circuitry for the gates of transistors Q.sub.A and Q.sub.B includes a current source I and a complementary transistor pair including transistors Q.sub.C and Q.sub.D, each of which has its drain shorted to its gate. A current source I supplies a current I.sub.3 to transistors Q.sub.C and Q.sub.D as well as an N-channel transistor Q.sub.E. Transistors Q.sub.C and Q.sub.D are typically much smaller than transistors Q.sub.A and Q.sub.B and act as current mirrors for transistors Q.sub.A and Q.sub.B, respectively. The output of a high-gain differential amplifier A is delivered to the gate of transistor Q.sub.E. As the output of differential amplifier A varies, the gate drive circuitry (including transistors Q.sub.C, Q.sub.D and Q.sub.E) causes an output current I.sub.OUT to vary in such a way that an output voltage V.sub.OUT is produced across the load resistor R.sub.L. For example, as the high-side transistor Q.sub.A is turned progressively on, the low-side transistor Q.sub.B is turned progressively off, so that the high-side current I.sub.1 exceeds the low-side current I.sub.2. The output current I.sub.OUT =I.sub.1 -I.sub.2 thus flows from the positive voltage rail V+ through resistor R.sub.L to ground. Conversely, as the high-side transistor Q.sub.A is turned progressively off, the low-side transistor Q.sub.B is turned progressively on. The high-side current I.sub.1 then falls below the low-side current I.sub.2, and the output current flows from ground through resistor R.sub.L to the negative voltage rail V-.
Amplifier 10 presents several problems. First, ideally transistor Q.sub.C should be perfectly matched to transistor Q.sub.A, and transistor Q.sub.D should be perfectly matched to transistor Q.sub.B. Otherwise, amplifier 10 will not have a stable and predictable quiescent current (i.e., the current that flows through transistors Q.sub.A and Q.sub.B when I.sub.OUT =0), and dead zones or overlap can occur when the input voltage is near the zero point. In FIG. 1B, the solid line shows a dead zone which occurs when both transistor Q.sub.A and transistor Q.sub.B are nonconductive (i.e., I.sub.1 =I.sub.2 =0) in some interval around V.sub.IN =0; and the dashed line shows the overlap which occurs when both transistor Q.sub.A and transistor Q.sub.B are conductive at V.sub.IN =0.
An article by F. Mistlberger and R. Koch ("Class-AB High-Swing CMOS Power Amplifier", IEEE Journal of Solid State Circuits, Vol. 27, No. 7, July 1992, pp. 1089-1092) describes a circuit for solving this problem, shown in simplified form in FIG. 2A, which in effect displaces the dead zones out of the critical low amplitude region, producing a waveform of the kind shown in FIG. 2B. This solution requires three differential amplifiers, however, and is complex.
A second problem arises because the gates of transistors Q.sub.A and Q.sub.B can never be biased beyond the voltage rails V+ and V-, respectively. Thus, for example, as transistor Q.sub.A becomes more conductive, and its source voltage approaches V+, the source-to-gate voltage V.sub.GS of transistor Q.sub.A is limited and transistor Q.sub.A can not be driven into the triode or resistive region. Similarly, the gate of transistor Q.sub.B cannot be pushed below V-, and this prevents transistor Q.sub.B from being driven into the triode region.
This effect is illustrated in FIG. 1C, which shows the variation of V.sub.OUT as V.sub.IN oscillates between V+ and V-. It will be noted that V.sub.OUT is clipped as V.sub.IN approaches the positive and negative voltage rails.
A known solution to this problem is shown in FIG. 3A, wherein a charge pump arrangement is used to boost the voltage at the gate of transistor Q.sub.A. In this arrangement, the positive terminal of current source I is connected to a common node between a bootstrap capacitor C.sub.B and a diode D.sub.B. Capacitor C.sub.B and diode D.sub.B are connected in series between the output of the amplifier and V+. Bootstrap capacitor C.sub.B charges as V.sub.OUT falls, with a current being drawn through diode D.sub.B. As V.sub.OUT increases, the voltage across capacitor C.sub.B remains roughly constant in the short term, and the positive terminal of current source I and the gate of transistor Q.sub.A are driven above V+. This is illustrated in FIG. 3B, where V.sub.BOOST is the voltage at the common node between diode D.sub.B and capacitor C.sub.B and V.sub.GATE is the gate voltage of transistor Q.sub.A. As is evident in FIG. 3B, the gate reaches a level above V+, and accordingly the V.sub.GS of transistor Q.sub.A is not limited and the clipping of V.sub.OUT is eliminated. As will be apparent, a similar charge pump can be connected to the source of transistor Q.sub.E to drive the gate of transistor Q.sub.B below the negative voltage rail V-.
Although the circuitry shown in FIG. 3A overcomes the clipping problem described above, it is not without disadvantages. The bootstrap capacitor C.sub.B must be fairly large because it must support the current flowing through the current source I and the transistors Q.sub.C, Q.sub.D and Q.sub.E. Thus, assuming that the amplifier is formed on an IC chip, capacitor C.sub.B must typically be fabricated as a discrete element. This requires an additional pin on the chip.
An additional disadvantage of the amplifier 10 shown in FIG. 1A is that is contains a P-channel output transistor Q.sub.B. As is well known, P-channel transistors must be larger than N-channel transistors to carry an equivalent current. Thus, assuming that the amplifier is integrated, the use of a P-channel output transistor sacrifices valuable chip area which could otherwise be devoted to other purposes.
These problems are overcome in an output stage according to this invention.